1) Field of the Invention
The present invention relates to an anaerobic purification device for purification of influent, such as wastewater, the anaerobic purification device comprising:
a reactor tank;
inlet means for introducing influent into the tank, the inlet means being located in the lower section of the tank;
water collecting means, such as an overflow gutter, for collecting purified water, the water collecting means being provided at the upper section of the tank and defining a liquid surface in said reactor tank;
at least one gas collecting system for collecting gas from the fluid contained in the reactor, the at least one gas collecting system being arranged at a level below the water collecting means;
a gas-liquid separation device arranged at a level above the water collecting means;
at least one riser having a discharge opening debouching into the separation device, the at least one riser being connected to the at least one gas collecting system for raising fluid contained in the tank by gas lift action caused by gas collected in the at least one gas collection system;
a downer having an inlet debouching into the separation device and an outlet debouching in the lower section of the tank for returning liquid separated in the separation device, into the lower section of the tank.
2) Description of Related Art
Such a device is known from EP-A-170.332. According to this EP-A-170.332 one subjects wastewater that contains organic material to a process in which dissolved organic material is broken down under anaerobic conditions. By contact with biomass that contains methane-producing microorganisms, methane is being produced which is separated from the liquid. The treated water (effluent) is removed via overflows weirs. EP-A-170.332 describes as starting point for that invention on page 1 lines 21-32: It has been found that with a residence time of several hours a purification of as much as 90% can be reached. The extent to which such purification efficiency can be maintained over a long period also depends on the sludge retention In particular, care must be taken to ensure that on average no more sludge is rinsed out of the reactor than can be formed in a certain period of time. If a high hydraulic flow is used with a low COD concentration in the influent, there is a considerable risk that the internal settler will not be capable of preventing a large amount of sludge being flushed out. A factor which is of importance in this connection is the hydraulic surface loading of the settler. In the subsequent passage, EP-A-170.332 explains that the upward flowing water and the rising gas bubbles can stir up the biomass flocks and particles considerably. These can arrive into the uppermost part of the reactor where the gas collecting system is located. The turbulence produced can thus result in excessive quantities of biomass to be flushed out of the reactor. This limits the loading capacity of the reactor considerably.
The invention of EP 170 332 aims to overcome the disadvantages just described and to create a reactor in which the main gas load is taken away from an uppermost gas collecting system. For this purpose EP 170 332 provides at least one additional gas collecting system for collecting gas, which additional system is arranged at a distance below the upper collecting system. The additional system has a hydraulic link with at least one riser pipe for raising liquid by gas lift action, said riser pipe discharging into at least one separation device for separating gas and liquid. In view of the fact that gas is trapped at a considerable distance below the liquid level and is conveyed further via the riser pipe, an essentially turbulence-free flow can occur in the upper section of the reactor. This increases the loading capacity, whereas at the top, clean effluent is obtained. It is important that the liquid, which is carried along with the gas to the riser pipe, is separated and returned to the reactor: While a quiet, eddy-free flow is required at the top of the reactor, very good mixing of sludge and fluid is required at the bottom of the reactor. For this purpose the heavy sludge near the bottom has to be fluidized. In a preferred embodiment according to EP 170 332 this fluidization can be achieved in the bottom section of the reactor with the aid of energy obtained from the gas lifting liquid in the riser pipe. The lifted liquid is separated from the gas and, under influence of hydraulic gravity pressure, returned from the separation device, through a downer pipe, to the bottom section of the reactor chamber.
For economic reasons, it is becoming more and more interesting to make the reactor column as high as possible. In that case, there would be more reactor volume and more biomass, whereas the footprint—the square meters of surface area occupied by the reactor—is the same. On the other hand, the higher the reactor the heavier the column of biomass in the reactor will be. The heavier the column of biomass, the more difficult it will be to maintain a good mixing and fluidization pattern near the bottom of the reactor. In some cases it may also happen that the biomass mixture becomes heavier due to the precipitation of inorganic material. Also in that case, it can be difficult to maintain a good fluidization.
A solution could be increasing the head pressure. However, prior art and experience teaches that, for a good mixture at the bottom of the reactor and overall functioning of the reactor, one requires, at the level of the liquid surface in the reactor, a head pressure of about 0.8 to 1 m water column (i.e. about 0.08-0.1 bar) in the downer, in order to overcome the pressure loss, which is required for good distribution at the bottom in the sludge bed. Too low head pressures result in non-optimal mixing at the bottom of the reactor and/or a poorer performance of the reactor respectively the ‘process carried out in the reactor’ as a whole, whereas too high head pressure would result in very high shear forces on the biomass particles, and consequently destruction of the granular material.
In practice at least about 80% of the head pressure is obtained from hydraulic pressure, whilst at most about 20% of the head pressure is obtained from gas pressure resulting from gas load situations during use. However in particular cases this has lead to problems with fluidization of the sludge in the bottom of the reactor and/or quite irregular gas flows.
Thus although for economic reasons one would like to make the reactor column as high as possible, the reactor height is in practice limited, because of the effects and teaching just mentioned.